This invention relates to flow fields for membrane electrode assemblies in electrochemical cells such as fuel cells. The flow fields of the present invention comprise a multitude of micro-flow channels preferably separated by similarly sized land features, where the channels preferably bear microstructured features. In fuel cells, the flow fields of the present invention provide enhanced distribution of fuel, improved water management, and improved electrical and thermal diffusion.
U.S. Pat. No. 5,108,849 concerns a 7-channel serpentine flow field design wherein 7 serpentine channels run in parallel. The serpentine design makes the channels 5-15 times longer than the actual dimension of the active part of the flow field plate. However, this increases the pressure drop across the plate. Known serpentine flow fields are also prone to reactant depletion along the channels, blanketing (which occurs when a stagnant layer of depleted gas acts as a diffusional barrier) due to the length of the channels and relatively low reactant residence times, and tenting (which occurs when the DCC material expands into the flow field plate channel). In a recent theoretical study, Yi and Nguyen (J. S. Yi, T. V. Nguyen. Proc. 1st Internat. Symp. PEM Fuel Cells. Eds. S. Gottesfeld, G. Halpert and A. Landgrebe, pp. 66-75, 1995) have also shown that for classical serpentine flow fields difficulties with diffusion of reactants under the land areas can have a significant negative effect on maximum achievable current densities from a given fuel cell.
McElroy (U.S. Pat. No. 4,855,193), Wilson (U.S. Pat. No. 5798187), and Zawodzinski et al. (C. Zawodzinski, M. S. Wilson, S. Gottesfeld. Proc. 1st Internat. Symp. PEM Fuel Cells. Eds. S. Gottesfeld, G. Halpert and A. Landgrebe, pp. 57-65, 1995) have evaluated the addition of a square weave metal screen type of flow distributor to the conventional serpentine flow design. These flow fields may comprise wire fabrics or screens wherein the wires form a series of coils, weaves, crimps or other undulating contours. This approach has several shortcomings, including non-uniform reactant distribution, water management problems, and corrosion.
Similarly, U.S. Pat. No. 5,641,586 concerns a bilayer structure wherein a porous layer overlays a layer having interdigitated flow channels.
The present invention provides an improved flow field for an electrochemical cell comprising one or more micro-flow channels having a depth or width (or preferably both) of less than 800 xcexcm, preferably less than 650 xcexcm, more preferably less than 250 xcexcm, and most preferably between 125 and 250 xcexcm. These micro-flow channels preferably have a pitch of less than 800 xcexcm, preferably less than 650 xcexcm, more preferably less than 250 xcexcm, and most preferably between 125 and 250 xcexcm. Optionally, the micro-flow channels of the invention further comprise micro-features within the channels whose dimensions in depth and width are less than 80% of the depth and width of the channel, preferably less than 50%, and more preferably less than 20%. Preferably the microfeatures have dimensions of less than 160 xcexcm and more preferably less than 100 xcexcm. Microfeatured flow fields may have channels of up to 3 mm in depth or width and still retain advantages of the present invention. In a preferred embodiment, these micro-flow channels comprise a highly parallel pattern which may contain interconnections or branch points.
In another aspect, the invention provides flow field plates comprising micro-channel flow fields according to the present invention.
In another aspect, the invention provides diffusion current collectors (DCC""s) comprising micro-channel flow fields according to the present invention.
In another aspect, the invention provides fuel cells comprising micro-channel flow field plates according to the present invention or DCC""s comprising micro-channel flow fields according to the present invention.
Flow fields of the present invention can achieve more uniform reactant distribution, more uniform pressure distribution, and improved water management in a fuel cell stack via the use of highly parallel micro-flow channels with additional smaller features for enhanced performance. In addition, the flow fields of the present invention allow for a decrease in thickness of the flow field plate and/or DCC, allowing a reduction in stack weight, volume, cost, and internal electrical resistance.
As used herein,
xe2x80x9cdiffusion-current collectorxe2x80x9d or xe2x80x9cDCCxe2x80x9d means a layer in an electrochemical cell adjacent to the active catalytic sites which allows transport of reactant and product mass and electric current to and from the active sites, which is preferably a porous electrically conductive material;
xe2x80x9chighly parallelxe2x80x9d means comprising many elements having the same function, in particular, having many channels connecting the same inlet to the same outlet;
xe2x80x9cunbranched aspect ratioxe2x80x9d is the ratio of the length of an unbranched channel segment to its hydraulic radius;
xe2x80x9chydraulic radiusxe2x80x9d is the cross-sectional area of a channel divided by the length of the perimeter of that cross section, e.g., the hydraulic radius of a circular channel is one-fourth its diameter; and
xe2x80x9cflow fieldxe2x80x9d, refers to a component of an electrochemical cell allowing ingress and egress of fluids such as reactant and waste gasses and liquids to and from reaction zones.
It is an advantage of the present invention to provide a flow field for improved fuel cell performance resulting from improved water and reactant gas distribution.